Astronomers detect a nearby gamma-ray burst-associated kilonova

Smoothed Gemini/NIRI Kband image of GRB 211211A at 4.1 days post-breakout. Astronomers detected a point source K ≈ 22.4 mag at the position of the optical afterglow of GRB 211211A. Credit: Rastinejad et al, 2022

An international team of astronomers has detected a new kilonova associated with a nearby gamma-ray burst (GRB) known as GRB 211211A. The discovery, reported in an article published April 22 on, could improve our understanding of the origin and nature of the still-mysterious GRBs.

Kilonovae (also called r-process supernovae) are transient events that occur when two compact objects, such as neutron stars, merge together. They are thought to emit short gamma-ray bursts and strong electromagnetic radiation due to the radioactive decay of heavy r-process nuclei. To date, kilonovae are the only observed source of r-process nucleosynthesis in the universe and may be responsible for the creation of the majority of elements heavier than iron.

GRB 211211A was identified on December 21, 2021 by the Burst Alert Telescope (BAT) aboard NASA’s Swift spacecraft, at a distance of approximately 1.14 billion light-years. It lasted about 51.37 seconds and its spectral harshness was found to be close to the long RGB population average. The light curve of this burst consists of several superimposed pulses showing little spectral evolution.

A group of researchers led by Jillian Rastinejad of Northwestern University in Evanston, Illinois, conducted a multi-wavelength tracking observation campaign of GRB 211211A to better understand its nature. To this end, they used instruments such as the Nordic Optical Telescope (NOT), the Calar Alto Observatory or the Karl Jansky Very Large Array (VLA).

Optical imaging of this GRB revealed an uncharted source that faded rapidly during the first three days after the burst. Complementary K-band observations with the Gemini-North telescope, source detected with a K-band luminosity of 22.4 mag, indicating a strong infrared excess over the optical afterglow curve. Subsequently, NOT imaging performed 17 days after the burst identified an associated supernova (SN).

The results suggest that this SN is indeed a kilonova. The researchers found that the supposed fusion had ejected about 0.04 solar masses of r-process rich material. This is consistent with the merger of two neutron stars with masses close to 1.4 solar masses.

“If we assume that the progenitor binary consists of two neutron stars and use the predictions from the merger simulations to constrain the relative masses and velocities of the components, we get a good fit with a binary of 1.4 + 1.3 solar masses producing ≈ 0.02 solar masses of ejecta, although matching the brightness on the first day may require additional heating by the GRB jet on the one-minute time scale of the burst,” explained the astronomers.

According to the authors of the paper, their detection of a kilonova after a long GRB implies that current neutron star merger rates calculated from short GRBs may underestimate the true population. They speculate that fusions related to long-lived GRBs can contribute significantly, both to the fusion rate of compact objects and to the enrichment of r-processes.

The unfolding story of a kilonova told in X-rays

More information:
JC Rastinejad et al, A Kilonova following a long duration gamma ray burst at 350 Mpc. arXiv:2204.10864v1 [astro-ph.HE]

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